Ilza !

: The Oxford English Dictionary has "swan" as a verb, labeled U.S.
slang, derived probably (it says) from northern England dialectal
"Is' wan," literally "I shall warrant" = I'll be bound; later taken
as a mincing substitute for "swear." The first use in print recorded
in the dictionary is from the year 1823.

Ilza, Are you moonlighting from the Poetry Archives? Does Stephen
know?